Controlled overgrowth of Pd on Au nanorods

Abstract

In this paper, we present a detailed, systematic study of the controlled overgrowth of Pd on Au nanorods. Pd nanoshells with fine-controlled dimensions and architectures were overgrown on single-crystalline Au nanorods through seed-mediated growth using H₂PdCl₄ as the Pd precursor and ascorbic acid as the reducing agent in the presence of cetyltrimethylammonium chloride (CTAC) or cetyltrimethylammonium bromide (CTAB) as the surface capping agent. The effects of surface capping agent, ascorbic acid to H₂PdCl₄ ratio, reaction temperature, and structure-directing foreign ions, such as Ag⁺, on the dimensions and architectures of the resulting Au@Pd core–shell heteronanostructures were systematically studied. At 30 °C, Au nanorods coated with polycrystalline Pd shells were obtained using CTAC as the surface capping agent, while single-crystalline Au@Pd core–shell nanocuboids formed in the presence of CTAB. The thicknesses of the polycrystalline and single-crystalline Pd shells were fine-controlled by adjusting the molar ratio of ascorbic acid to H₂PdCl₄. At an elevated reaction temperature of 60 °C, irregularly shaped and cylindrical co-axial core–shell nanorods were obtained in CTAC and CTAB, respectively. By introducing Ag⁺ ions into the Pd growth solution, Au nanorods coated with segregated Pd nanoislands and dumbbell-like core–shell heteronanostructures were obtained as a consequence of the underpotential deposition of Ag and sustainable galvanic replacements that concurred during the Pd overgrowth processes. The effects of the Pd shell dimensions and morphologies on the plasmonic properties of the Au@Pd core–shell nanostructures were also investigated.